This invention relates in general to the reduction in drag over an aircraft engine nacelle and more particularly, but not by way of limitation, to an arrangement for the maintenance of laminar flow over a nacelle surface having gaps in the surface.
Roughly half of the drag an aircraft experiences in flight is due to skin friction. Great efforts are expended in attempting to reduce drag because of the fuel savings resulting from any reduction. It has been estimated that about 4% of the drag experienced by aircraft using gas turbine engines mounted on the aircraft through a wing or fuselage mounted pylon results from freestream flow of air over the engine nacelle.
A large number of designs have been developed by engineers seeking to reduce aircraft and nacelle drag. Nacelles have been shaped to provide maximum natural laminar flow, such as the design described by Lahti, et al in U.S. Pat. No. 4,799,633.
Attempts have been made to blow air out through holes in an aircraft wing from within the wing to reduce drag, such as is described by Fleischmann in U.S. Pat. No. 2,873,931. Axially directed ridges have been placed on an aerodynamic surface to direct air flow in a manner to reduce drag, as disclosed by Rethorst, in U.S. Pat. No. 3,588,005. Mechanisms within an aircraft wing have been provided to change the airfoil shape during flight to optimize the wing for flight conditions, e.g., cruise, take-off and landing, as described by Readnour et al in U.S. Pat. No. 5,000,399. While these systems often reduce drag somewhat, the improvement tends to be limited to smooth surfaces, since surface irregularities such as skin gaps or recesses often cause local transition from laminar to turbulent flow.
A great variety of structures involving sucking air inwardly through a porous aerodynamic surface have been developed and endeavor to reduce drag by maintaining laminar flow along the aerodynamic surface. Typically, Dannenberg in U.S. Pat. No. 3,128,973 shows wing panels having a porous surface through which air can be drawn into the wing interior. Glaze shows, in U.S. Pat. No. 3,056,432, permeable woven wire material forming longitudinal portions of an aircraft wing skin. It is also known to provide longitudinal slots in an aircraft wing through which air can be drawn into the wing interior. Although these systems may beneficially encourage laminar flow over small areas or along longitudinal lines, problems remain with obtaining uniform inward flow over large areas. Also, many of the prior systems for drawing air into a wing or the like present a rough surface or surface discontinuities that will tend to increase drag. Further, these systems are useful only over large, smooth surfaces, such as aircraft wings, and cannot accommodate turbulence inducing gaps in the surfaces, such as the inherent gaps around movable doors or panels.
Rose, et al describe, in U.S. Pat. No. 4,479,150, a boundary layer control system for use in a nacelle that provides a layer of honeycomb sound suppression material on the internal surface of the nacelle skin. A porous skin made up of a fine woven mesh is adhesively bonded to a perforated aluminum sheet. The honeycomb sound suppression system is bonded to the opposite side of the aluminum sheet. Suction headers engage the inside surface of the honeycomb, with an impermeable skin between headers. Honeycomb walls are partially cut-away to permit airflow through the honeycomb cell walls and the perforated sheet to reach the headers. While generally effective, this system tends to be primarily useful over the smooth, forward portions of an engine nacelle. Once the laminar flow reaches the interface between the aft edge of the nose cowl and the forward edge of fan cowl, with the inherent irregular gap therebetween, or access doors and the like with peripheral gaps or recesses, the laminar flow quickly deteriorates into turbulent flow.
Thus, there is a continuing need for a system for eliminating the interference with laminar flow that results from skin surface gaps or recesses, especially where other suction systems are used over the smooth forward portion of the nacelle. While laminar flow often continues well aft of the aft end of an active surface suction system, any significant surface irregularity in the extended area can cause immediate transition to turbulent flow.